Wal-Mart, the largest US retailer, on Tuesday announced the launch of a music download service that sells songs from EMI and Universal Music Group without the digital rights management (DRM) copy protection that limits the kinds of device on which they can be played.

The move, which comes as Amazon is planning a similar service, marks a potential challenge to the digitial dominance of Apple’s iTunes, since it offers music that can be played both on Apple’s iPod devices and on less popular but cheaper MP3 players.